ABSTRACT In all eukaryotic cells intracellular membrane trafficking is critical for a range of important cellular functions including protein secretion, post-translational modifications, cell signaling, cell polarization, and cell maintenance. Defects in membrane trafficking can underline, or even exacerbate, a number of human diseases including cancer, diabetes mellitus, Alzheimer's, cystic fibrosis, Hermansky-Pudlak syndrome and Congenital Disorders of Glycosylation. We have pioneered the functional analysis of the Conserved Oligomeric Golgi (COG), an evolutionarily conserved complex of eight gene products, each of which is critical for the membrane trafficking in the Golgi apparatus. The COG complex interacts with SNAREs, SM proteins, Rabs, coiled-coil tethers and COPI coat to organize specific docking and fusion of transport intermediates with their acceptor membrane. At least half, but probably all COG subunits consist largely of helical bundles stacked in series to form long flexible rods. This architectural feature is also found in three related tethering complexes Dsl/ZW10, GARP and exocyst that act to tether vesicles to ER, TGN and plasma membrane, suggesting a common underlying tethering mechanism. This proposal seeks to probe in greater detail the mechanism by which the COG complex orchestrates trafficking in the Golgi complex. 1. We will test the hypothesis that membrane bound COG subunits are arranged in several combinations including Lobe A, Lobe B and Lobe A/B complexes that are spatially separated and likely to participate in different membrane trafficking steps. 2. We will test the hypothesis that the COG4 and COG8 proteins can initiate the formation of two different tethering platforms which attract two populations of Golgi trafficking intermediates. 3. We will test the hypothesis that the interactions between COG subunits and their protein partners are necessary to maintain the COG complexes function of tethering retrograde vesicles recycling Golgi enzymes and retrograde cargo. 4. We will use biochemical and microscopy approaches to investigate cross-talk between the COG complex and other CATCHR complexes specifically focusing on the cross-talk between Lobe B of the COG complex and the GARP complex, both of which are required for the assembly of trans-Golgi SNARE complex.